The present invention pertains to medication dispensing devices, and, in particular, to a portable medication dispensing device such as an injector pen.
Patients suffering from a number of different diseases frequently must inject themselves with medication. To allow a person to conveniently and accurately self-administer medicine, a variety of devices broadly known as injector pens or injection pens have been developed. Generally, these pens are equipped with a cartridge including a piston and containing a multi-dose quantity of liquid medication. A drive member, extending from within a base of the injector pen and operably connected with typically more rearward mechanisms of the pen that control drive member motion, is movable forward to advance the piston in the cartridge in such a manner to dispense the contained medication from an outlet at the opposite cartridge end, typically through a needle that penetrates a stopper at that opposite end. In disposable pens, after a pen has been utilized to exhaust the supply of medication within the cartridge, the entire pen is discarded by a user, who then begins using a new replacement pen. In reusable pens, after a pen has been utilized to exhaust the supply of medication within the cartridge, the pen is disassembled to allow replacement of the spent cartridge with a fresh cartridge, and then the pen is reassembled for its subsequent use.
One known type of injection pen uses a pull/push sequence applied to an externally accessible plunger member to deliver medication from the pen. The pulling of the plunger member in an axial, proximal direction first cocks or arms the pen for dose delivery, and then the axial pushing of the plunger member in the distal direction advances the cartridge piston to dispense the medication. While useful, such devices are not without their shortcomings. For example, some users who possess limited hand strength may find it difficult to apply sufficient force to push in the plunger member to inject a dose. In addition, the short plunger member travel associated with delivering very small doses in some pens may cause some users to question whether in fact a pen is operating to delivery the expected relatively small dose.
A number of injection pens are disclosed in International Publication No. WO 03/080160, the disclosure of which is incorporated herein by reference. These injection pens are advantageously designed with a mechanical advantage to allow lesser required plunger forces and greater plunger travel. However, when such pens offset the gear set from the axis of the cartridge, an asymmetrical injection pen may result if the overall pen size is minimized as is often desirable. Still further, the high mechanical advantages afforded by such pens may, under certain operating conditions, result in high forces being experienced by the internal gears or other components, which therefore may need to be made more robust to account for such forces.
Thus, it would be desirable to provide an apparatus that can overcome one or more of these and other shortcomings of the prior art.